1. Field of the Invention
The present invention relates to a wired board (i.e., a board provided with interconnecting conductors or wiring) for use as, for example, a BGA (ball grid array) package base for IC (integrated circuit) packages and a plastic or resinous printed board (i.e., a board consisting of a substrate made of a resinous material) such as a motherboard and adapted to mount thereon the above described BGA package base or the like.
2. Description of the Related Art
A conventional wired board for a BGA package base includes a substrate made of an electrically insulating material such as alumina ceramic and a number of connection terminals or bumps formed on the main surface of the substrate. An example of such connection terminal is shown in FIG. 15 and generally indicated by 7'. Each connection terminal 7' includes a mass 5' of solder and a solder ball 6' bonded to a bonding pad 3' by way of the mass 5' of solder. The bonding pad 3' is formed on a main surface 2a' of a substrate 2' for metallizing the same and treated by a predetermined plating process. The mass 5' of solder consists of Pb-Sn eutectic solder or the like solder which is relatively low in melting point (hereinafter also referred to as low melting point solder). The solder ball 6' is made of solder which is relatively high in melting point (hereinafter also referred to as high melting point solder) and contains a high percentage of lead (Pb), as for example PB90-Sn10. The solder ball 6' is bonded to the plated surface of the bonding pad 3' by way of the mass 5' of solder, whereby to constitute the connection terminal 7'. In use, the wired board 1' is mounted on a printed board having bonding pads corresponding in arrangement to those of the wired board 1' in such a manner that their connection terminals are respectively aligned with each other, and then their terminals are bonded to each other, whereby the wired board 1' is electrically connected to the printed board. FIG. 16 shows an assembly of a wired board 1' for a BGA package base having such connection terminals 7' shown in FIG. 15 and a printed board 21' which are soldered together at their bonding pads 3' and 23'.
Such an assembly encounters the following problems. Namely, an assembly of a ceramic wired board or the like which is low in coefficient of thermal expansion and a plastic or resinous printed board which is high in coefficient of thermal expansion which are soldered together at their bonding pads (hereinafter also referred to simply as pads) has the possibility that due to the difference in expansion and contraction between the wired board 1' and the printed board 21' resulting from the difference in coefficient of thermal expansion between them a crack or cracks K are caused at the soldered interconnection portions (hereinafter also referred to as BGA interconnection portions) which are located between the bonding pads 3' and 23' as shown in FIGS. 17 and 18 to cause disconnection. This is because the solder is relative low in strength and is incapable of resisting or enduring the thermal stress or shearing stress parallel to the main surfaces of the wired board 1' and the printed board 21'. The stress becomes maximum at or adjacent an interface between each bonding pad 3' or 23' and the mass of solder.
The low melting point solder has the property of being hard to deform since it is more brittle and less ductile or malleable as compared with the high melting point solder. In addition, at the interface between the mass of low melting point solder and the bonding pad is formed an intermetallic compound of Au-Sn or Ni-Sn, which is hard and brittle, due to the diffusion caused between the Au-plated or Ni-plated layer on the bonding pad and tin (Sn) in the mass of solder. For this reason, when a large temperature variation occurs after assembly, initiation and growth of a crack or cracks K is liable to be caused in such a manner that a crack or cracks K initiate at a point on the outer periphery of each mass 5' of solder and extend therefrom along the main surfaces 2a' and 22a' of the substrates 2' and 22', while leaving solder layers of quite small thicknesses T1 and T2 on the respective pads 3' and 23', i.e., leaving a single thin solder layer on each of the pads 3' and 23' as shown in FIG. 18. Once such cracks K are caused, they are likely to be developed throughout the mass 5' of solder from the initiation point at one side to the other side at one time, thus causing the possibility that defective conduction or disconnection between the pads 3' and 23' is incurred.
This problem becomes more prominent as the difference in coefficient of thermal expansion between the materials of the substrates for the wired board and the printed board becomes larger and as the boards become larger in size. So, depending upon the size of the boards, there may occur such a problem that a ceramic wired board cannot be mounted on a plastic printed board. Further, there may be incurred such a problem that depending upon the material of one of the mating boards the material of the other is restricted or the size of the wired board cannot be increased.
In the meantime, the cracks between the pads naturally occur not only on the wired board 1' side but on the printed board 23' side in the similar manner as shown in FIGS. 17 and 18. However, in case, for example, a ceramic wired board and a plastic printed board are bonded together, there is a tendency that cracks are actually caused in the masses of solder at or adjacent the interface between each pad on the ceramic package base side and the corresponding mass of solder and scarcely at the interface between each pad on the plastic printed board and the corresponding solder. This is for the following reasons. The pad constituting the connection terminal of the ceramic wired board is comprised of a plated layer of several microns in thickness formed on a metallized layer of tungsten or the like, whereas the pad on the plastic or resinous printed board is made of copper and so thin, i.e., 20 to 30 .mu.m, and above all things copper is considerably softer as compared with tungsten.
Accordingly, in case the pads of the wired board and the printed board are soldered together, the pads on the printed board side are thick and soft in itself so they are liable to deform to absorb the above described thermal stresses resulting from a temperature variation, whereas the pads on the ceramic wired board side are hard and thin so they can be hardly expected to effect such a stress absorbing action. For this reason, in many cases, cracks are caused at or adjacent the interface between the ceramic board side pad and its associated mass of solder.